The down-hole seismic explosive sources currently in use, as shown diagrammatically in FIG. 1, generally comprise a metal part PM terminating a cable linked to the source and used for inserting it into the borehole. An explosive charge SE is disposed on the metal part and is connected to a detonator which is itself connected to ignition control circuits via electrical conductors contained in the connecting cable which constitutes a logging cable.
Thus, for a borehole of given section, the explosion of the explosive charge CE takes place in the liquid medium constituted by drilling mud. The mechanical energy dissipated by the explosion in the fluid filling the upper portion of the borehole is not negligible, and consequently there is a corresponding reduction in the mechanical energy which exerts pressure normal to the wall of the borehole, regardless of whether the borehole is lined with casing TU or not. The diameter of a bubble created in a liquid by an explosion falls off with increasing pressure and is inversely proportional to the cube root of the hydrostatic pressure which, in the above-mentioned conventional situation, is the pressure existing at the depth at which the source is used. The coupling of the mechanical energy released by the explosion to the side walls of the borehole and enabling said mechanical energy to be transferred normally to the walls of the borehole and thence into the adjacent geological strata under investigation, therefore falls off with source depth, which is highly inconvenient when performing successive tests down a single borehole.
The object of the present invention is to remedy the above-mentioned drawback by providing a down-hole seismic source with containment.